Hemodynamics - Lecture 16

Chapter Links and Quizzes

 

Hemodynamics

Determinants of Resistance

Poiseuille Equation

Viscosity of blood

Audio - Lecture -

Hemodynamics - study of blood flow

study of the interrelationships of blood pressure, blood flow, and the physical properties of blood

blood (non-Newtonian) is not like water (Newtonian) - if true then easy to understand blood consists of plasma and cells - cells interact and give blood some unique properties

Remember Ohm's law V = IR; DP = QR

in a single tube (blood vessel)

Q = DP/R

Determinants of Resistance

1. length - not important because constant

2. radius

3. viscosity of fluid (maple syrup vs. water)

4. # of vessels (lanes on parkway)

1842 Poiseuille developed equation for flow:

        DP p r 4

Q = ----------------

          8 l h

viscosity -- eta

important to see that most important factor is r : Q = r⁴;

for resistance --- because Q = DP/R

          8 l h

R = ----------------

         p r 4 #

where # is the number of blood vessels

then it can be concluded that the most important factor is R = 1/r⁴

if radius is doubled, flow increases 16 times

arterioles most important - change diameter the most

Diameter most important - this is value that changes most in circulation, but viscosity is also important.

# of vessels doesn't change drastically (exercise - increase # of capillaries)

Audio - Lecture -

Viscosity depends on

1. shear rate - relative velocity of 2 adjacent fluid layers

2. hematocrit:

increase Hct - increase viscosity

3. tube size - related to hematocrit

4. cell flexibility

  increase flex - decrease viscosity

sickle cell disease - increase viscosity

compensates by decrease hct - i.e. anemia

water 1.0 centipoise

plasma 1.6 cp

blood 4.0 cp

When flow falls (decrease Pa) velocity falls --- this causes a decrease in SR

more interactions between RBC's

viscosity of blood increases - stacking of RBC's (stack of coins - rouleaux)

Other determinants of viscosity:

Tube size effects hematocrit - small vessels

Fahraeus Effect

- accept this, if you want to know, study your Physics - Bernoulli's Law

How can this be?

-RBC's travel in center stream where velocity is fastest

only plasma along wall

more plasma than red blood cells in small vessels - thus reduced hematocrit

phenomenon is known as Fahraeus Lindqvist Effect

As a result of these two conditions, the viscosity throughout the network is relatively constant during normal conditions - this can change during pathophysiology

REVIEW QUESTIONS

1. Which of the following will not cause an increase in vascular resistance
   1. increased vessel length
   2. decreased viscosity
   3. increased hematocrit
   4. decreased number of blood vessels
   5. decreased shear rate
2. Which of the following procedures would cause the greatest increase in blood flow through a cylindrical tube?
   1. increase tube diameter by 10%
   2. decrease the tube length by 20%
   3. decrease the blood viscosity by 30%
   4. increase the driving pressure by 40%
3. A decrease in shear rate from 100 to 1 sec^-1 will cause _________.
   1. viscosity of 40% blood to decrease
   2. viscosity of plasma to decrease
   3. blood cells to form rouleaux
   4. vascular resistance to decrease
   5. None of the above are correct.
4. Which of the following is incorrect?
   1. As vessel diameter increases from 20 mm to 100 mm the hematocrit increases.
   2. As hematocrit increases from 20% to 30% the viscosity increases.
   3. As vessel diameter decreases from 3 mm to 2 mm the viscosity decreases.
   4. The shear rate is the lowest in the capillaries.
   5. The viscosity in the arterioles is not significantly different from the viscosity in the aorta.
5. Which of the following is a Newtonian Fluid.
   1. 45% hemaotcrit blood
   2. plasma
   3. 0% hematocrit blood
   4. water
   5. More than one of the above are correct.

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